Camila Cabello and Shawn Mendes have been having a great time in New York City for his 21st birthday. And while they haven't officially announced that they're a couple, Cabello greeted her rumored boyfriend with a sweet declaration of love.

"Happy birthday to this magic human, I love you!!!!❤️❤️❤️❤️," she posted on Instagram to commemorate Mendes' special day (August 8).

The "Senorita" collaborators have been putting their flouring romance on display with all the PDA. And clips from his birthday bash, appropriately hashtagged "#SM21," show a very happy Mendes giving Cabello a kiss.

Cabello told Variety about how much he means to her. "I’ve known Shawn for such a long time and it’s so much fun getting to work and do things with somebody who means a lot to you," she told the magazine.

And since they have known each other for meeting on Austin Mahone's tour in 2014, sources say that even though their relationship is "moving quickly," the timing is great for them.

"It already feels to them like they've been together for a lot longer than they have," one of the sources told E! News. "They've known each other and been a part of each other's lives for a while so it's all very natural and moving quickly."